As a power supply device that lights an LED element as a light-emitting element, there is a power supply device having a dimming function for receiving a dimming signal having a predetermined dimming degree from the outside and adjusting an optical output of an LED according to the dimming degree.
As the power supply device having such a dimming function, there is a power supply device including constant-current controlling means for controlling an electric current flowing to an LED element to be constant and constant-voltage control means for controlling a load voltage applied to the LED element to be constant. The power supply device is enabled to perform stable lighting control for the LED element in a wide dimming degree region by switching constant current control and constant voltage control according to a dimming degree and changing a ratio of the controls to subject LED element to the constant current control in a dimming degree region with a shallow dimming degree, i.e., a dimming degree region with a large optical output of the LED element and subject the LED element to the constant voltage control in a dimming degree region with a deep dimming degree, i.e., a dimming degree region with a small optical output of the LED element.
However, in the power supply device in the related art, when the LED element is in a lighting state with an optical output having a certain dimming degree, if a dimming signal having a deepest dimming degree equivalent to extinction of the LED element is input, the LED element is instantaneously extinguished from the lighting state at the immediately preceding dimming degree. Therefore, it is likely that a user present in a lighting space is given a sense of discomfort.